The little LED flashlights are a great source for lights. How many batteries were yours to use? The one's I have had used three AAAs. At 1.5 volts each that comes to 4.5 volts. I've used LED units with the white wire from china girl engines with success (light flickers, but that's OK with me). The white wire puts out roughly 6 volts and did not ever burn out any of the LED units I've used for headlights or tail lights.
With my four strokes I haven't had the benefit of a white wire to work with, so have used old sidewall generators putting out 6 volts. Used to be I was always burning out bulbs at speed as I think they were putting out over 6 volts at 30 or more mph and incandescent bulbs seem pretty sensitive to anything more than what they were designed for. The little LEDs seem to be more tolerant of a wider voltage range. How tolerant, I don't know, but I can say that I have had good luck so far running head lights and tail light, brake lights using said LED flashlight units designed to be run on 4.5 volts, but in my case powered by 6 volt sidewall generators probably putting out as much as 8 or 9 volts at 40 or so mph. So that's good! I've been altering big Harbor Freight dune buggy lights to use as headlights and making my own copper jewel lights for tail, brake and fender running lights. (Type in "copper jewel light" on google and it should give you a link.
I am certainly no expert on lighting, electronics or any of this. I am, however, a resourceful cheapskate who likes to see at night and more importantly BE SEEN AT NIGHT by otherwise distracted, drugged, drunk, texting and sometimes comatose drivers.
Depending on what voltage batteries you plan to use I doubt that you will need resistors for six volt input, but might be pushing your luck with 12 volts. As inexpensive as these little flashlights are I'd just try it with six volts and see what happens. Oops isn't going to break the bank. Good luck! Let there be light!
SB